Reperfusion arrhythmias: role of early afterdepolarizations studied by monophasic action potential recordings in the intact canine heart during autonomically denervated and stimulated states

Incorporating inductances in tissue-scale models of cardiac electrophysiology

In standard models of cardiac electrophysiology, including the bidomain and monodomain models, local perturbations can propagate at infinite speed. We address this unrealistic property by developing a hyperbolic bidomain model that is based on a generalization of Ohm's law with a Cattaneo-type model for the fluxes. Further, we obtain a hyperbolic monodomain model in the case that the intracellular and extracellular conductivity tensors have the same anisotropy ratio. In one spatial dimension, the hyperbolic monodomain model is equivalent to a cable model that includes axial inductances, and the relaxation times of the Cattaneo fluxes are strictly related to these inductances. A purely linear analysis shows that the inductances are negligible, but models of cardiac electrophysiology are highly nonlinear, and linear predictions may not capture the fully nonlinear dynamics. In fact, contrary to the linear analysis, we show that for simple nonlinear ionic models, an increase in conduction velocity is obtained for small and moderate values of the relaxation time. A similar behavior is also demonstrated with biophysically detailed ionic models. Using the Fenton-Karma model along with a low-order finite element spatial discretization, we numerically analyze differences between the standard monodomain model and the hyperbolic monodomain model. In a simple benchmark test, we show that the propagation of the action potential is strongly influenced by the alignment of the fibers with respect to the mesh in both the parabolic and hyperbolic models when using relatively coarse spatial discretizations. Accurate predictions of the conduction velocity require computational mesh spacings on the order of a single cardiac cell. We also compare the two formulations in the case of spiral break up and atrial fibrillation in an anatomically detailed model of the left atrium, and we examine the effect of intracellular and extracellular inductances on the virtual electrode phenomenon.

Effects of dantrolene on arrhythmogenicity in isolated regional ischemia-reperfusion rabbit hearts with or without pacing-induced heart failure

Dantrolene was reported to suppress ventricular fibrillation (VF) in failing hearts with acute myocardial infarction, but its antiarrhythmic efficacy in regional ischemia-reperfusion (IR) hearts remains debatable. Heart failure (HF) was induced by right ventricular pacing. The IR rabbit model was created by coronary artery ligation for 30 min, followed by reperfusion for 15 min in vivo in both HF and non-HF groups (n = 9 in each group). Simultaneous voltage and intracellular Ca²⁺ (Ca_i) optical mapping was then performed in isolated Langendorff-perfused hearts. Electrophysiological studies were conducted and VF inducibility was evaluated by dynamic pacing. Dantrolene (10 μM) was administered after baseline studies. The HF group had a higher VF inducibility than the control group. Dantrolene had both antiarrhythmic (prolonged action potential duration (APD) and effective refractory period) and proarrhythmic effects (slowed conduction velocity, steepened APD restitution slope, and enhanced arrhythmogenic alternans induction) but had no significant effects on ventricular premature beat (VPB) suppression and VF inducibility in both groups. A higher VF conversion rate in the non-HF group was likely due to greater APD prolonging effects in smaller hearts compared to the HF group. The lack of significant effects on VPB suppression by dantrolene suggests that triggered activity might not be the dominant mechanism responsible for VPB induction in the IR model.

In vivo recording of monophasic action potentials in awake dogs

Assessment of cardiac repolarization in dogs in vivo is of interest in numerous experimental canine models. Previous studies have used monophasic action potentials (MAPs) to investigate repolarization in vitro and in vivo in anesthetized animal models. Therefore, an approach for recording MAPs in awake dogs without the interference of anesthesia is desirable. We describe an experimental technique to record MAPs in conscious dogs by means of conventional rubber introducers which are implanted into the internal jugular vein. Atrial as well as ventricular MAPs can be simultaneously measured without complications. Pacing thresholds are low and stable over time. Continuous MAP recordings of stable amplitude can be achieved from the same endocardial site for periods up to 1h. Antegrade and retrograde atrioventricular nodal conduction properties can be assessed. Programmed stimulation can be performed to simultaneously determine local refractory periods and MAP duration at cycle lengths from 500 to 200ms. In awake, unsedated dogs measuring MAPs via rubber introducers permits safe, long-term recording of MAPs. Such recordings may be useful for safety pharmacologic studies in evaluating cardiovascular and noncardiovascular drugs with respect to their effects on repolarization. In various canine in vivo models including in vivo models of long QT syndrome, heart failures or sudden cardiac death, the present technique permits electrophysiologic measurements without the interference of anesthesia.

Experimental study of the effect of autonomic nervous system on the transmural dispersion of ventricular repolarization under acute myocardial ischemia in vivo

The effect of the autonomic nerves on the transmural dispersion of ventricular repolarization (TDR) under acute myocardial ischemia in intact canine was investigated. Using the monophasic action potential (MAP) recording technique, MAPs of the epicardium (Epi), mid-myocardium (Mid) and endocardium (Endo) were recorded simultaneously by specially designed plunge-needle electrodes at the left ventricular free wall under acute myocardial ischemia in 12 open-chest dogs. MAPD90 and TDR among three myocardial layers as well as the incidence of the early afterdepolarization (EAD) before autonomic nervous stimulation and during autonomic nervous stimulation were compared. It was found that 10 min after acute myocardial ischemia, TDR was increased from 55 +/- 8 ms to 86 +/- 15 ms during sympathetic stimulation (P < 0.01). The TDR (53 +/- 9 ms) during parasympathetic stimulation was not significantly different from that of the control (55 +/- 8 ms) (P > 0.05). The EAD was elicited in the Mid of 2 dogs (16%) 10 min after acute myocardial ischemia, but the EAD were elicited in the Mid of 7 dogs (58%) during sympathetic stimulation (P < 0.01). It was concluded that: (1) Sympathetic stimulation can increase the transmural dispersion of repolarization and induce early afterdepolarizations in the Mid under acute myocardial ischemia, which provide the opportunity for the ventricular arrhythmia developing; (2) Parasympathetic stimulation has no significant effect on the transmural dispersion of repolarization under myocardial ischemia.

A K(ATP) channel opener inhibited myocardial reperfusion action potential shortening and arrhythmias

Low concentrations of certain K(ATP) channel openers have been reported to exert a moderate inhibitory effect on arrhythmias during post-ischaemic early myocardial reperfusion, but the accompanying effects on the time course of changes in action potentials in intact hearts have not yet been studied. We report that in rat isolated hearts, reperfusion following 10 min of regional no-flow ischaemia was associated with both an acute, marked, but transient, shortening of ventricular repolarisation (by 63%) during reperfusion, and a high incidence (90%) of ventricular tachyarrhythmias. The K(ATP) channel opener Ro 31-6930 [2-(6-cyano-2,2-dimethyl-2H-1-benzopyran-4-yl)-pyridine 1-oxide], delivered prior to ischaemia at a relatively low concentration (0.5 microM), significantly reduced the incidence and duration of reperfusion arrhythmias, and prevented the associated acute action potential shortening during reperfusion, each in a glibenclamide (1 microM)-sensitive manner (P<0.05, n=10-15 hearts). This was associated with a moderate and non-arrhythmogenic action potential shortening during ischaemia (a potentially "cardioprotective" effect). However, these data highlight the potential harm these drugs may cause, since a higher concentration of Ro 31-6930 caused marked shortening of action potentials and significant pro-arrhythmia during ischaemia.

Myocardial ischemia-reperfusion damage impacts occurrence of ventricular fibrillation in dogs

To define the relationship between ischemia-reperfusion-induced myocardial damage (IRD) and the occurrence of ventricular tachycardia (VT) and fibrillation (VF), we studied 23 dogs with a three-dimensional activation mapping system. Left anterior descending (LAD) coronary artery occlusion and reperfusion were performed while recording electrograms during VF and atrial pacing. Prior nonischemic sites showing IRD, defined as at least 10% loss of electrogram voltage after reperfusion, had the longest ventricular effective refractory periods (ERPs). IRD sites also occurred more frequently in dogs with reperfusion VF (44 +/- 2 sites, P < 0.01) compared with dogs with VT (18 +/- 5 sites) and no VT (16 +/- 3 sites). In dogs (n = 3) with 3 h of reperfusion, 95% of IRD sites still had lower voltage than those recorded during occlusion. Activation mapping of the first eight complexes of VF had Purkinje or endocardial focal origin in 57%, and complexes originated from IRD sites in 28%. In contrast, dogs with only reperfusion VT also had Purkinje or endocardial focal origin in 79%, but only 5% (P < 0.01 vs. VF dogs) of the sites of origin had IRD. Therefore, dogs with reperfusion VF had more IRD sites where the ERP was longest, and more focal ventricular complexes originated from IRD sites, indicating that IRD may be one important factor in the occurrence of VF during reperfusion.

Cardiac ionic currents and acute ischemia: from channels to arrhythmias

The aim of this review is to provide basic information on the electrophysiological changes during acute ischemia and reperfusion from the level of ion channels up to the level of multicellular preparations. After an introduction, section II provides a general description of the ion channels and electrogenic transporters present in the heart, more specifically in the plasma membrane, in intracellular organelles of the sarcoplasmic reticulum and mitochondria, and in the gap junctions. The description is restricted to activation and permeation characterisitics, while modulation is incorporated in section III. This section (ischemic syndromes) describes the biochemical (lipids, radicals, hormones, neurotransmitters, metabolites) and ion concentration changes, the mechanisms involved, and the effect on channels and cells. Section IV (electrical changes and arrhythmias) is subdivided in two parts, with first a description of the electrical changes at the cellular and multicellular level, followed by an analysis of arrhythmias during ischemia and reperfusion. The last short section suggests possible developments in the study of ischemia-related phenomena.

Extracellular discontinuities in cardiac muscle: evidence for capillary effects on the action potential foot

It has become of fundamental importance to understand variations in the shape of the upstroke of the action potential in order to identify structural loading effects. One component of this goal is a detailed experimental analysis of the time course of the foot of the cardiac action potential (Vm foot) during propagation in different directions in anisotropic cardiac muscle. To this end, we performed phase-plane analysis of transmembrane action potentials during anisotropic propagation in adult working myocardium. The results showed that during longitudinal propagation there was initial slowing of Vm foot that resulted in deviations from a simple exponential; corollary changes occurred at numerous sites during transverse propagation. We hypothesized that the effect on Vm foot observed in the experimental data was created by the microscopic structure, especially the capillaries. This hypothesis predicts that the phase-plane trajectory of Vm foot will deviate from linearity in the presence of a high density of capillaries, and that a linear trajectory will occur in the absence of capillaries. Comparison of the results of Fast and Kléber (Circ Res. 1993;73:914-925) in a monolayer of neonatal cardiac myocytes, which is devoid of capillaries, and our results in newborn ventricular muscle, which is rich in capillaries, showed drastic differences in Vm foot as predicted. Because this comparison provided experimental support for the capillary hypothesis, we explored the underlying biophysical mechanisms due to interstitial electrical field effects, using a "2-domain" model of myocytes and capillaries separated by interstitial space. The model results show that a propagating interstitial electrical field induces an inward capacitive current in the inactive capillaries that causes a feedback effect on the active membrane (source) that slows the initial rise of its action potential. The results show unexpected mechanisms related to extracellular structural loading that may play a role in selected conduction disturbances, such as in a reperfused ischemic region surrounded by normal myocardium.

Electrophysiologic effects of interactions between activated canine neutrophils and cardiac myocytes

INTRODUCTION

Myocardial ischemia causes neutrophils to bind to activated myocytes and liberate platelet-activating factor (PAF). PAF causes delayed repolarization, early afterdepolarizations (EADs), and arrest of repolarization. We studied the effect of activation of neutrophils bound to canine cardiac myocytes to determine if such activation causes PAF generation and similar changes in transmembrane potentials.

METHODS AND RESULTS

Myocytes from canine left ventricle and neutrophils from the same dog were superfused with Tyrode's solution and transmembrane potentials recorded from the former. Neutrophils (100 microL, 10(6)/mL) were added and allowed to bind to the myocytes. Neutrophils were activated with 1% zymosan-activated serum (ZAS). CV-6209 (100 nM) was used to block receptors for PAF. Liberation of PAF by activated neutrophils was quantified with a commercial radioimmunoassay kit. Neutrophils activated with ZAS caused changes in myocyte transmembrane potentials like those induced by PAF: action potential prolongation, runs of EAD, and periods of plateau arrest. PAF receptor blockade prevented neutrophil activation from altering transmembrane potentials. Neutrophils activated with 1% ZAS liberated significant amounts of PAF.

CONCLUSIONS

When neutrophils bound to cardiac myocytes are activated by exposure to 1% ZAS, they cause prompt and consistent changes in myocyte electrical activity that could be arrhythmogenic for the in situ heart. These changes are similar to those caused by PAF in pharmacologic studies. Neutrophils activated in this manner generate PAF, and the effects of their activation are prevented by blockade of PAF receptors. We conclude that, during reperfusion of ischemic myocardium, PAF generated by activated neutrophils most likely is a cause of some arrhythmias.

Arrhythmias caused by platelet activating factor

INTRODUCTION

Both ischemia and reperfusion are associated with ventricular arrhythmias. In both instances, neutrophils migrate into the ischemic zone, are activated by locally released factors, and bind to myocytes. The activated neutrophils liberate platelet activating factor (PAF). We have studied the arrhythmogenic actions of PAF on transmembrane potentials of isolated canine cardiac myocytes.

METHODS AND RESULTS

Cardiac myocytes were prepared from normal canine hearts by standard methods and studied in vitro by recording transmembrane potentials under control conditions and during exposure to graded doses of PAF, usually 0.25 to 1.25 micrograms (0.25 to 1.2 microM). Myocytes were superfused with Tyrode's solution (2.0 mL/min), paced at a cycle length of 1000 msec, and maintained at a temperature between 36 degrees and 38 degrees C. PAF caused a consistent and dose-dependent set of alterations in the transmembrane potential, including increased action potential duration, runs of early afterdepolarizations (EADs), and transient arrest of repolarization (PA). In addition, in some myocytes PAF caused intermittent small depolarizations both at the plateau voltage and resting potential. The effects of PAF were transient: only some residual action potential prolongation was noted after Tyrode's washout for 5 minutes. Effects of PAF were blocked in a dose-dependent manner by the PAF receptor antagonist, CV-6209. Both tetrodotoxin (1.2 x 10(-6) M) and xylocaine (5 x 10(-5) M) antagonized the ability of PAF to cause EADs and PA.

CONCLUSIONS

PAF consistently exerts arrhythmogenic effects on the membrane of ventricular myocytes. Since PAF is liberated by activated neutrophils and since activated neutrophils migrate into ischemic myocardium on reperfusion, we judge that PAF liberated by such neutrophils is an important arrhythmogenic factor for reperfusion arrhythmias. The same mechanism may be a cause of arrhythmias during the evolution of infarction.